The most common were no eye & ear protection, shooting
against unknown backstops & targets.
Hmm, aren't this quite a bit of the norm for self defense shootings?
Look, lack of eye and ear protection are not the concerns of the antis and if they have ever been mentioned as the concern, then it would have to be in an extremely low minority of the arguments. I know I have never seen the antis claiming that guns are bad or should not be owned because people don't wear eye and ear protection. It is the safety oriented shooters that care about eye and ear safety, not the antis.
As for unknown backstops and targets, in many cases that I have seen, your term of "unknown" is just that. As the viewer, you are only able to see what is within the immediate view of the camera. So just because something is unknown to you does not necessarily mean it is unknown to the shooter.
I have never seen any complaints that guns should be banned because somebody has fired them under water. Do you really think this is significant?
Yes, there are a lot of idiots on Youtube in EVERY category from sewing to cooking to shooting to flying aircraft. In shooting, there is plenty to complain about that would upset the antis, but eye and ear protection and shooting underwater aren't really salient issues. Negligent discharges, shooting toward other people, illegal acts, etc. would be much more salient to complain about.